1. Field of the Invention
The present invention relates to controlling the state of focus of a light beam in respect to a record medium surface in an optical reproducing apparatus, and more particularly is directed toward circuitry which controls the preliminary state of focus of a light beam in respect to a disc in an optical reproducing apparatus.
2. Description of the Prior Art
In an optical reproducing apparatus, such as a digital audio disc player, a signal is recorded on a record medium surface, such as a disc, by forming a plurality of pits thereon which are optically readable by an optical pickup device. Typically, the optical pickup device includes an object lens for focusing a light beam on the record medium surface and at least one photodetector which produces a focus error signal indicating the state of focus of the light beam on the record medium surface. Such focus error signal is supplied to a servomechanism which, during reproduction of the optically recorded signal, positionally controls either the entire optical pickup device or the object lens contained therein to properly focus the light beam on the record medium surface.
However, in order for the servomechanism to effectively maintain an acceptable state of focus of the light beam in respect to the record medium surface in response to the focus error signal, the object lens must be within a predetermined range of positions relative to the record medium surface. Thus, a preliminary focusing operation is required to initially position the object lens within that predetermined range. In accordance therewith, an auxiliary focus control circuit generally is provided and supplies a preliminary focus signal to the servomechanism such that following the preliminary focusing operation the beam of light impinging on the record medium surface can be maintained in an acceptable state of focus in response to the focus error signal supplied to the servomechanism.
A typical previously proposed auxiliary focus control circuit is shown in FIG. 1 to include a low frequency oscillator 10 which generates a sine wave or triangular wave voltage having a low frequency and serving as a preliminary focus signal. The preliminary focus signal is supplied, during the preliminary focusing operation to a focus control device, such as an electro-magnetic linear motor 50, through a switch 30 and a focus driving circuit 40. Focus driving circuit 40 typically includes an operational amplifier 41 which receives the preliminary focus signal at its non-inverting input. As is characteristic of circuitry associated with an operational amplifier, resistances R.sub.FD1, R.sub.FD2 and R.sub.FD3 are respectively shown connected between the operational amplifier inverting input and ground, between the operational amplifier inverting input and output, and between the operational amplifier non-inverting input and ground. As is well known in the art, the term "ground" is understood to mean earth potential. An output signal from focus driving circuit 40 is supplied to the coil of linear motor 50. In accordance with the value of the preliminary focus control signal, focus control device or motor 50 varies the position of an optical pickup device 60 or an object lens 61 contained therein. That is, either the entire optical pickup device 60 or object lens 61 is repositioned by focus control device 50 in response to the preliminary focus control signal, supplied by low frequency oscillator 10, such that a beam of light supplied from optical pickup device 60 is properly focused in respect to a surface of a record medium, such as a disc 100.
Variations in the focusing state of the light beam in respect to record medium surface 100 are graphically illustrated in FIG. 2. More specifically, curve C represents variations in the focusing state of the light beam impinging on record medium surface 100 and the distance between the broken lines L.sub.1 and L.sub.2 represents the normal range of control possible by focus control device 50 in response to a focus error voltage e.sub.f supplied to focus drive circuit 40 from a photodetector 62 contained in optical pickup device 60. Portions of curve C which are above and below a mid-line L respectively correspond to underfocused and overfocused states of the light beam in respect to record medium surface 100. As the focusing condition varies from an underfocused to overfocused state, curve C crosses line L at point A and focus error voltage e.sub.f varies from a negative value to a positive value and passes through a zero crossover point corresponding in time to point A. Additionally, as the focusing condition of the light beam in respect to record medium surface 100 varies from an overfocused state to an underfocused state, curve C crosses point B and focus error voltage e.sub.f varies from a positive value to a negative value passing through a zero crossover point corresponding in time to point B. Both points A and B on curve C represent a substantially focussed state of the light beam on record medium surface 100.
Referring once again to FIG. 1, focus error voltage e.sub.f is shown to be supplied to both a focus discriminating circuit 70 and terminal or fixed contact 32 of switch 30. Additionally, switch 30 is connected to the output of focus discriminator circuit 70. Focus discriminating circuit 70 is designed to detect the zero crossover point of focus error voltage e.sub.f corresponding to point B on curve C. When such a zero crossover point occurs, that is, when focus error voltage e.sub.f changes from a positive value to a negative value, an output signal S.sub.2 of focus discriminating circuit 70 varies from a low level to a high level and thereby causes switch 30 to switch its movable contact from engagement with its fixed contact or terminal 31 to engagement with its contact or terminal 32. Thus, in response to a high level of output signal S.sub.2, switch 30 terminates the preliminary focusing operation by no longer supplying the output signal of low frequency oscillator 10 to focus driving circuit 40. That is, focus error voltage e.sub.f replaces the preliminary focus signal as the input signal to focus driving circuit 40. The light beam from optical pickup device 60 is now within a controllable range such that focus control device 50 can maintain an acceptable focus state of the light beam on record medium surface 100 by using focus error voltage e.sub.f as the input signal to focus driving circuit 40.
However, as disclosed previously, the auxiliary focus control circuit of FIG. 1 varies the position of optical pickup device 60 or object lens 61 in response to the sine wave voltage or triangular wave voltage of low frequency oscillator 10. Thus, as shown in FIG. 2, in the preliminary focusing operation, the variation in the focusing condition can begin from a just underfocussed state at point D, on curve C, to a very underfocussed state at point E, to a very overfocussed state at point F before reaching point B. Such unnecessary variations in focusing of the light beam are due to the different values of the sine wave or triangular wave voltage which can be assumed at the beginning of the preliminary focusing operation inasmuch as the phase of the sine wave or triangular wave is not fixed at the beginning of a preliminary focusing operation. Therefore, the time interval from the beginning of the preliminary focusing operation to the time when the preliminary focusing operation ends at the point B on curve C can vary undesirably. In particular, the time to complete the preliminary focusing operation can extend up to approximately the period of the sine wave or triangular wave voltage which, for a typically low oscillating frequency of 0.75 hertz (hz), is over one second.